In mobile telecommunication systems, wireless or radio access networks, RAN, provide radio based access for mobile terminals to a communication network. Therein, frequency resources are provided for radio communication between a mobile terminal and the access network, which then relays the communication to the communication network the mobile terminal wishes to access, and/or which is associated with the RAN.
These frequency resources may be licensed, i.e. certain frequency resources may be exclusively used by a certain network operator, or unlicensed, i.e. certain frequency resources may be used freely by anyone. An example for licensed frequency resources are those used by mobile communication networks, specifically radio access networks, according to 3GPP standards, e.g. GSM, UMTS/(W)CDMA, or LTE.
An example for unlicensed frequency resources are those used by wireless networks according to IEEE 802.11 standards, also denoted WiFi or WLAN, or according to the Bluetooth standard.
One example for communication networks as mentioned above are Public Land Mobile Networks (PLMN), i.e. networks that are established and operated by an administration or by a recognized operating agency (ROA), also called network operator, for the specific purpose of providing land mobile telecommunications services to the public.
PLMNs usually provide defined and reliable services and capabilities, so-called carrier-grade features. One such carrier-grade feature is the capability for lawful interception in the country where the mobile terminal is accessing the network. This is e.g. guaranteed for circuit-switched (CS) voice calls according to the 3GPP standards as defined in 3GPP TS 33.106 V12.0.0 and TS 33.107 V12.2.0.
A further service that may be offered by a PLMN is the IMS (IP Multimedia Subsystem) telephony service, as for example defined in 3GPP TS 22.173 V12.3.0. In order to be rated as carrier-grade, this service would have to offer telephony features comparable to those of CS voice call services.
PLMNs are usually accessed by radio access networks according to the 3GPP standards as mentioned above. However, today an increasing demand for alternative mobile accesses occurs, such that for example WLAN access to 3GPP networks is provided. This is for example defined in the 3GPP TS 24.234 V11.3.0 and IEEE 802.11u standards.
As the WLAN network spectrum is unlicensed, anyone may operate a WLAN access point more or less at any location. Consequently, a WLAN access point may be operated by a network operator not having licenced spectrum resp. frequency resources available in a particular country. This network operator could thereby provide access to selected services in the home PLMN (HPLMN), e.g. in the IMS, or generally to the Internet. For example an American operator may operate WLAN access points in Germany, and by this enabling that the subscribers of this American operator roaming in Germany can access their home communication network in USA, resp. certain services offered therein, via a WLAN access point in Germany.
In such a case, certain carrier-grade features, for example lawful interception, can be provided by the operator of the WLAN access point on IP level. However, when the mobile terminal of the subscriber uses carrier grade IMS voice service, the lawful interception may no longer be successful as encrypted signalling, for example SIP signalling, would be sent between the mobile terminal and operator network nodes located in the home country (e.g. USA). Further, if media plane security is used, the encrypted voice media would be sent between the mobile terminal and operator network nodes located in the home country. Thus, authorities in the country or area of the WLAN access would be unable to execute lawful interception of the carrier grade IMS voice calls of the mobile terminal.